Instability analysis for a centrifugal pump with straight inlet pipe using partially averaged Navier–Stokes model

Abstract

The current study numerically investigates the flow instability under several part-load conditions in a centrifugal pump with a straight inlet pipe to explore the underlying relationship between a positive slope phenomenon and internal flow using a partially averaged Navier–Stokes model. The model was validated by comparing the hydraulic performance and averaged flow in the impeller between the numerical results and experimental data of a tested pump. The internal flows in pumps have been intensively investigated based on Batchelor vortex family, Rayleigh–Taylor criterion, entropy generation rate, and energy equation to analyze the flow instability from different aspects. The simulation results using partially averaged Navier–Stokes model are acceptable due to the good agreement with the experimental data for the tested pump. No matter the geometry of the inlet pipe, the pre-swirling flows in the inlet pipe are in the convective instability region. Under the part-load condition of φ = 0.5 φbep, the axial vorticity coefficient is affected by the geometry of the inlet pipe. However, under the part-load condition with rotating stall, e.g. φ = 0.78 φbep, the flow in the inlet pipe is affected by the unstable flow in the pump impeller. For the pump with a straight inlet pipe, the vortex inside the blade-to-blade passage is in a stable state according to Rayleigh–Taylor criterion under the condition of φ = 0.5 φbep. However, the vortex in the blade-to-blade passage is in an unstable state due to centrifugal instability under those operation conditions with rotating stall cells in the impeller, and the dominant oscillations are dependent on the propagation of rotating stall cells. Finally, head loss analysis based on energy equations elucidates that turbulent kinetic energy production term is predominant in the head loss in pump impeller. The present results are helpful for better understanding of the unstable flows and positive slope phenomenon for centrifugal pumps.